The overall goal of this project is to understand the function of nitric oxide (NO) in organism development. NO is a diffusible multifunctional second messenger implicated in numerous physiological functions in animals. Recently NO has emerged as an important factor in cell proliferation, cell differentiation, and animal development. Our studies have shown that NO acts as an antiproliferative agent during organism development, regulating the balance between cell proliferation and differentiation, and, ultimately, controlling the shape and size of tissues and organs in the developing Drosophila. This proposal will test the hypothesis that NO is a general regulator of cell proliferation and differentiation during organism development and morphogenesis. We will combine genetic and molecular approaches to study the mechanisms of NO action. We will search for mutations in the dNOS locus. Such mutations will provide a starting point to develop specific genetic lesions in dNOS gene and to perform screens for genetic partners of dNOS. We will characterize the molecular organization of the isolated NO synthase (NOS) genes involved in Drosophila development, and we will search for other NOS gene(s) in Drosophila genome. We will elucidate the biological functions of the developmentally regulated isoforms which are capable of inhibiting NOS activity, and we will search for other genes that interact with NOS. Recently, we have demonstrated that NO interacts with the retinoblastoma pathway to control eye development in Drosophila, and here we propose to study genetic and molecular interactions between NOS and the components of the cell cycle machinery.